Generally, wind power generating systems convert kinetic energy created by wind into electric energy.
Wind turbines have been used for decades to exploit the energy of the wind e.g. to produce electricity. To reduce the price of electricity produced by such wind turbines, the size of the wind turbines have increased to a current average nominal power of commercial wind turbines of approximately 1.5 MW, while wind turbines of up to 3 MW are under development, and it is expected that even larger wind turbines will be marketed in the coming years. Common commercial wind turbines have three blades, which by a 1.5 MW wind turbine have a length of approximately 35 m.
Typically, such a wind power generating system includes a tower which is supported on supporting ground, a nacelle which is provided on the upper end of the tower, and a blade which is coupled to the front end of the nacelle. The nacelle has therein several devices, such as a gear box, a generator, an inverter, etc., which are necessary to generate electricity. In the wind power generating system, the blade and the gear box convert kinetic energy created by wind into high-speed kinetic energy of 1500 rpm or more. The generator coupled to the gear box converts the high-speed kinetic energy into electric energy.
Here, in the case of electronic elements installed in the nacelle, the performance thereof may be lowered after they have been used for a predetermined period of time. Some of the elements may require replacement. Particularly, in the case where the wind power generating system is used offshore, abrupt malfunction may be induced by water, salinity, etc., so that an emergency repair may be required.
Generally, a wind turbine includes a rotor having multiple blades. The rotor is mounted on a housing, or nacelle, that is positioned on top of a truss or tubular tower. Utility grade wind turbines (i.e., wind turbines designed to provide electrical power to a utility grid) can have large rotors, e.g., 30 meters (m) (98 feet (ft)) or more in diameter. Blades, attached to rotatable hubs on these rotors, transform mechanical wind energy into a mechanical rotational torque that drives one or more generators. The generators are generally, but not always, rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the turbine rotor for the generator to convert the rotational mechanical energy to electrical energy, which is fed into a utility grid. Gearless direct drive turbines also exist.
Some examples of stress factors are vertical wind shear, localized turbulence (including interaction of the rotor with the tower), gravity, wind flow variations and start-stop cycles. Vertical wind shear is typically defined as the relationship between wind speeds and height above the surface of the earth, i.e., altitude. In general, as the altitude increases, wind speed increases. Given a blade of 30 meters (98 ft) or more, and the subsequent large diameter of rotation (at least twice the blade length plus the diameter of the associated hub), wind speed can increase 5% to 10% above that at the hub centerline from the hub centerline up to the end of the blade at the blade tip with the blade pointing straight upward. The wind speed may also decrease 5% to 10% below that at the hub centerline from the hub centerline down to the end of the blade at the blade tip with the blade pointing straight downward. As the blades rotate, the cyclic increasing and decreasing of the wind shear induces a cyclic stress within the blades.
Localized turbulence includes stationary wakes and bow waves induced by the blades and by the near proximity of the rotating blades to the tower. As the blades rotate through these localized regions, additional stresses are induced within the blades. Also, as the blades rotate, gravity induces fluctuating bending moments within the blades that also induce stresses. Cyclic acceleration and deceleration of the blades due to the aforementioned wind flow variations and start-stop cycles induce cyclic stresses on the blades as well.
The blades are typically designed and manufactured to withstand such stresses including the cumulative impact of such stresses in a variety of combinations. The blades are also designed and manufactured to withstand the cumulative impact of a predetermined number of stress cycles, commonly referred to as fatigue cycles. Upon exceeding the predetermined number of fatigue cycles, the potential for material delamination may increase.
As described above, blades are typically attached to a rotating hub at attachment regions designed and fabricated to receive the blades. The blades also have integral attachment regions. The hub and the blade attachment regions act as load transfer regions. For example, the weight of the blades and the aforementioned cyclic stresses are transferred to the hub attachment regions via the blade attachment regions.
With wind energy plants, the fastening of the rotor blades, which are subjected to considerable forces, to the shaft of the wind energy plant, which is coupled to the generator, is a general problem, since due to the forces acting upon the rotor the components used are exposed to extreme stresses. The structural form of the so-called rotor blade connection is thus of great significance.
The blades are subject to large forces and bending moments inter alia due to the wind pressure and due to the weight and rotation of the blades, and further the blades are subject to fatigue because of the cyclic load. For example, during one revolution, the blade travels through a region of maximum wind load in the upper part of the circle, whereas the blade experiences a low wind area (or even lee), when the blade passes the tower, and further the wind is normally not constant, as there may be gusts of wind. Naturally the root of the blade and the connection of the blade to the hub must be able to withstand the load of the blade, and a failure of the blade root or the hub would be devastating and potentially fatal to persons near the wind turbine.
With one known wind energy plant, the rotor blade, consisting of a compound material, is connected in the region of the so-called rotor blade base, i.e. in the rotor blade's end region that is to be coupled to the rotor hub, to a metallic flange that consists of an inner and outer ring flange; in this, the rotor blade base is set between the inner and the outer ring flange and is fastened by means of an adhesive and a threaded connection. For the producing of the threaded connection, a bolt is inserted through a passage bore, which passes completely through the rotor blade, into the rotor blade and screwed. The flange is screwed to the rotor hub at its end opposite to the rotor blade base. This construction of the rotor blade connection is relatively complicated and heavy structurally, since the metallic flange has a high weight. Especially disadvantageous is the fact that the rotor blade is significantly weakened in the region of the rotor blade base by the passage bore for receiving the bolt.
An additional, known wind energy plant displays as the rotor blade connection a steel flange joint, in which the rotor blade base is clamped between an inner and an outer flange and the two flanges are screwed together. The joining of the two flanges with the rotor hub takes place by means of a spaced-apart flange ring, with the aid of high-strength expansion screws. In this construction the metallic flanges contribute very often up to a third to the total weight of the rotor blade. Furthermore, the force progression is unfavorable, due to a radial offset between the rotor blade base and the spaced-apart flange, since this leads to an undesired lever effect.
In the case of the above mentioned experimental AEOLUS II, a so-called cross-bolt connection is used, in which the so-called cross bolts are arranged in passage bores in the region of the rotor blade base (i.e. the hub-side end region of the rotor blade), which passage bores are formed in the rotor blade and pass completely through the latter. The cross bolts arranged inside the passage bores are laminated into the rotor blade and serve as anchoring elements inside the rotor blade. The cross bolts are in each case connected to a tensioning element, formed as a bolt-shaped tension rod, that is screwed together with the rotor hub. By means of the tensioning element, which is subjected to tension, the rotor blade is pressed against the hub and thus held. In this construction it is likewise especially disadvantageous that the rotor blade is greatly weakened in the region of the rotor blade base by the passage bores for receiving the cross bolts. Furthermore, the force progression in the region of the flange-like rotor hub is unfavorable.
Therefore, there is a need for a rotor blade for use on a wind energy plant, in which the disadvantages of the prior art are to a large extent avoided, and which have an easily producible and secure connection between the rotor blade and the rotor hub, which connection is able to withstand extreme stresses.
There is a further need for a rotor blade for use on a wind energy plant, in which the disadvantages of the prior art are to a large extent avoided, and which have an easily producible and secure connection between the rotor blade and the rotor hub, which connection is able to withstand extreme stresses yet which can be easily and efficiently removed and replaced.
Over the years different approaches have been tried out, as can be seen in U.S. Pat. No. 4,915,590 that discloses a wind turbine blade attachment method. This prior art blade attachment comprises fibre glass sucker rods secured in the blade root, which sucker rods are unbonded to the blade root for a substantial portion forming a free end at the root end, and further the free end of the sucker rods are recessed from the blade root end, which means that the sucker rods can be put under tension. The patent indicates that the sucker rods may be unbonded to the rotor blade for approximately 85% of the length. The sucker rods are tapered down in diameter toward the secured end in the bonded area, where the rod is mated internally to the blade. Although this may be appropriate for relatively small blades used on wind turbines in August 1987, when this US-application was filed, this prior art construction is not suited, however, for the relatively large blades currently used, as the sucker rods will not be able to withstand the very large forces present at the blade root of large blades, especially as the rods are only bonded to the blade root to a very limited extent.
In the blade attachment of WO-A2-01/42647, the blade is connected to the hub by bolts screwed into inserts provided in radial holes in the blade root. It is a disadvantage however, that radial holes must be provided in the blade root, as these holes seriously weaken the construction and provides a stress concentration, which means that the blade root must be constructed to be very strong and hence heavy, which again stresses the construction.
A similar construction is described in U.S. Pat. No. 6,371,730, which discloses a blade connected to the hub by bolts screwed into nuts inserted into radial blind holes in the blade root. Although the holes are not through-going, they nonetheless seriously weaken the blade root, and hence this construction is also not advantageous.
It has also been tried to provide a blade root with fully bonded or embedded bushings each having a projecting threaded bolt part, as disclosed in U.S. Pat. No. 4,420,354. This prior art incorporates drilling a relatively large axially extending hole in the blade root made of a wood-resin composite, in which hole the bushing, having a preformed resin sleeve, is resin bonded. With this prior art a relatively large amount of blade root material is removed, which weakens the construction, so the blade root must be overdimensioned. Especially with large blades of modem composites like fibre-reinforced plastics, which are relatively flexible, stress concentration at the end of the bushings may be detrimental, as the bushings are significantly more stiff. Moreover this prior art method is somewhat destructive, and as fibre composites for the blade root are quite expensive, and increasingly will be as larger blades are developed, as it is expected that high-tech materials like carbon fibre composites will be introduced, this procedure is not favorable.
In general, prior art methods of the kind set forth are quite labor intensive and time consuming, as the bushings are spaced by blocks of e.g. a foam material, and the blocks and the bushings must be arranged carefully. Further there is a risk of air pockets being formed in the blade root between the bushings and the blocks, and such air pockets, which are difficult to detect, will seriously deteriorate the strength of the blade root.